As energy independence becomes more important, there is a need to replace petroleum with other sources of energy. These sources can include, for example, solar, wind, nuclear, coal, and biomass. Global coal resources exceed petroleum resources and are much more uniformly distributed geographically. However, conventional coal utilization technologies often result in undesirable environmental impacts.
Coal is a complex, heterogeneous material based on an insoluble macromolecular structure. Recovery of valuable chemical products from this starting material is challenging, due in large part to the heterogeneity and complexity of the coal structure and also due to its generally-accepted insoluble nature, which limits potential processing and “refining” options that are available for other feed stocks. Most traditional coal utilization strategies are based on recovery of energy, either directly as heat or more typically at a commercial scale, as electricity. Coal is mined, cleaned to remove as much inorganic material as is practical, and burned in one of a variety of combustion systems to generate heat used to produce steam, which is used for electrical generation.
Gasification involves conversion of coal to syngas (CO+H2) followed by utilization of the syngas for electrical generation (IGCC) or production of synthetic liquids (Fischer-Tropsch/indirect liquefaction). Liquid products can also be produced from coal by pyrolysis, often in conjunction with metallurgical coke production. At one time, the coal tar industry was a major source of raw chemical feedstocks, but declining demand for coke, the relatively low quality of coal tars (for refining purposes) and the heretofore low cost of petroleum-derived fuels have reduced commercial-scale production of chemical feedstocks from coal tars.
Direct liquefaction technology has been extensively explored. This utilization strategy was largely aimed at production of synthetic petroleum-substitutes for production of transportation fuels. The concept underlying this approach was conversion of coal to liquid products by limited disruption of the macromolecular structure of the coal by reductive bond breaking. This was typically accomplished by processing of the coal with H2, usually at high pressures and moderately high temperatures, often in conjunction with a carrier solvent and usually in conjunction with catalytic materials. This technology was never successfully demonstrated to be an economically viable route for conversion of coal to useful products and much of this effort was abandoned.
In addition to the technologies for converting coal into useful energy referred to above, there are processes involving supercritical water oxidation for destruction of wastes. A supercritical fluid is any fluid at a temperature and pressure above its thermodynamic critical point. Supercritical water oxidation is designed for complete oxidation of the organic material and is generally employed for the destruction of hazardous waste.
In addition to coal, studies have demonstrated the partial conversion of switchgrass to lower molecular products using subcritical water. At 235° C., it was found that up to 51.1% of the weight of the carbon present in the switchgrass could be converted. Kumar et al., Biocrude Production from Switchgrass Using Subcritical Water, Energy Fuels 23: 5151-5159 (2009). In addition, conversions up to 54.8% by weight of the initial mass at 250° C. and up to 78.9% by weight at 300° C. were obtained for switchgrass in a batch reactor. Cheng et al., Investigation of Rapid Conversion of Switchgrass in Subcritical Water, Fuel Process Tech. 90: 301-311 (2009).
Coal is a critical global resource in a time when energy independence is becoming more important. Global coal resources exceed petroleum resources by a factor of more than 10 and coal is much more uniformly geographically distributed that petroleum. However, utilization of coal and other organic solids is inhibited by the fact that they are solid and associated with a wide range of environmental problems. Prior technologies have failed to solve these problems. Therefore, an environmentally benign conversion of organic solids, such as coal, to a liquid that can be pumped, distilled, or otherwise processed by conventional liquid processing technology is a long-standing technological goal.